Use Harmony mode in uglifier for ES2015+ support

Recent versions of graphql-rails updated React to a version which
requires this mode so it can be compiled on production.

So we enable this mode is described in the Uglifier README [2].

[1] https://github.com/lautis/uglifier/tree/v4.2.0#es6--es2015--harmony-mode

config/environments/staging.rb

@@ -30,7 +30,7 @@ Rails.application.configure do
   config.public_file_server.enabled = ENV["RAILS_SERVE_STATIC_FILES"].present?
 
   # Compress JavaScripts and CSS.
-  config.assets.js_compressor = :uglifier
+  config.assets.js_compressor = Uglifier.new(harmony: true)
   # config.assets.css_compressor = :sass
 
   # Do not fallback to assets pipeline if a precompiled asset is missed.